Notice how Devil Rats are immune but carriers. Rats can't vomit, which is why they are so easily poisoned. I suspect that this may be the basis for the VITAS disease. But the key thing here is that it was 1) broad based virus and 2) it induced an allergic response. That means the viral mechanism can use a wide variety of hosts, which makes culturing very easy. It also means that it can fundamentally change the human immuno-response system. While its current version affects it adversely, that doesn't mean it can't be modified to have beneficial effects. Properly tailored, such a virus can be very useful and inexpensive to produce.